Note that the form factor relations as stated in Eqs. (REF ), () and () are model independent. Yet, in deriving the
above Ward identities [1]}, one assumes that the light-quark degrees of freedom can be neglected and hence takes
\(p=p_b\)  as suggested by heavy quark effective theory in the limit \(m_b \rightarrow \infty \)  with \(p_b/m_b\)  constant. This is a
sensible approach and justified in the case of \(B\)  mesons, though nonperturbative \(\Lambda _\mathrm {QCD}/m_c\)  corrections
due to the light quarks may be important in charmed mesons. As known from a series of nonperturbative studies, heavy quark effective
theory is not generally a good guide to charm physics [2]}, [3]}, [4]}, [5]}, [6]} since the charm quark is neither light nor really heavy. Eventually, a reliable calculation of
heavy-light form factors, couplings and decay constants requires the correct description of dynamical chiral symmetry breaking,
that is the effect of the light degrees of freedom.
<FIGURE><FIGURE>